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libgsmfr2: implement encoder homing
author Mychaela Falconia <falcon@freecalypso.org>
date Sun, 14 Apr 2024 03:06:03 +0000
parents fe5aceaf51e0
children 9208db14b4b9
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1 The external public interface to Themyscira libgsmefr consists of a single
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2 header file <gsm_efr.h>; it should be installed in the same system include
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3 directory as <gsm.h> from classic libgsm (1990s free software product) for the
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4 original FR codec, and the API of libgsmefr is modeled after that of libgsm.
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5
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6 The dialect of C we chose for libgsmefr is ANSI C (function prototypes), const
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7 qualifier is used where appropriate, and the interface is defined in terms of
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8 <stdint.h> types; <gsm_efr.h> includes <stdint.h>.
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9
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10 State allocation and freeing
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11 ============================
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12
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13 In order to use the EFR encoder, you will need to allocate an encoder state
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14 structure, and to use the EFR decoder, you will need to allocate a decoder state
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15 structure. The necessary state allocation functions are:
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17 extern struct EFR_encoder_state *EFR_encoder_create(int dtx);
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18 extern struct EFR_decoder_state *EFR_decoder_create(void);
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19
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20 struct EFR_encoder_state and struct EFR_decoder_state are opaque structures to
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21 library users: you only get pointers which you remember and pass around, but
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22 <gsm_efr.h> does not give you full definitions of these structs. As a library
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23 user, you don't even get to know the size of these structs, hence the necessary
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24 malloc() operation happens inside EFR_encoder_create() and EFR_decoder_create().
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25 However, each structure is malloc'ed as a single chunk, hence when you are done
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26 with it, simply call free() to relinquish each encoder or decoder state
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27 instance.
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29 EFR_encoder_create() and EFR_decoder_create() functions can fail if the malloc()
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30 call inside fails, in which case the two libgsmefr functions in question return
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31 NULL.
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33 The dtx argument to EFR_encoder_create() is a Boolean flag represented as an
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34 int; it tells the EFR encoder whether it should operate with DTX enabled (run
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35 GSM 06.82 VAD and emit SID frames instead of speech frames per GSM 06.81) or DTX
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36 disabled (skip VAD and always emit speech frames).
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37
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38 Using the EFR encoder
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39 =====================
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40
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41 To encode one 20 ms audio frame per EFR, call EFR_encode_frame():
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42
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43 extern void EFR_encode_frame(struct EFR_encoder_state *st, const int16_t *pcm,
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44 uint8_t *frame, int *sp, int *vad);
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45
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46 You need to provide an encoder state structure allocated earlier with
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47 EFR_encoder_create(), a block of 160 linear PCM samples, and an output buffer of
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48 31 bytes (EFR_RTP_FRAME_LEN constant also defined in <gsm_efr.h>) into which the
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49 encoded EFR frame will be written; the frame format is that defined in ETSI TS
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50 101 318 for EFR in RTP, including the 0xC signature in the upper nibble of the
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51 first byte.
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52
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53 The last two arguments of type (int *) are optional pointers to extra output
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54 flags SP and VAD, defined in GSM 06.81 section 5.1.1; either pointer or both of
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55 them can be NULL if these extra output flags aren't needed. Both of these flags
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56 are needed in order to test our libgsmefr encoder implementation against
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57 official ETSI test sequences (GSM 06.54), but they typically aren't needed
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58 otherwise.
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59
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60 Using the EFR decoder
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61 =====================
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62
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63 The main interface to our EFR decoder is this function:
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64
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65 extern void EFR_decode_frame(struct EFR_decoder_state *st, const uint8_t *frame,
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66 int bfi, int taf, int16_t *pcm);
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67
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68 The inputs consist of 244 bits of frame payload (the 4 upper bits of the first
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69 byte are ignored - there is NO enforcement of 0xC signature in our frame
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70 decoder) and BFI and TAF flags defined in GSM 06.81 section 6.1.1. Note the
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71 absence of a SID flag argument: EFR_decode_frame() calls our own utility
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72 function EFR_sid_classify() to determine SID from the frame itself per the rules
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73 of GSM 06.81 section 6.1.1.
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74
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75 The canonical EFR decoder always expects frame bits input to be present, even
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76 during BFI condtions! More specifically, if a BFI=1 decoding call comes in when
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77 the decoder is in comfort noise generation state (after a SID), then all frame
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78 bits passed along with BFI=1 are ignored as one would naturally expect for
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79 frames that typically aren't transmitted at all - but if a BFI=1 decoding call
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80 comes in when the decoder is in regular speech mode, the canonical decoder will
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81 use the "fixed codebook excitation pulses" part of the erroneous frame (one
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82 declared to be garbage) as part of its decoding operation! (This part
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83 constitutes 35 bits per subframe or 140 bits out of 244 per frame.)
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84
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85 BFI with no data
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86 ================
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87
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88 Many EFR decoder applications will be faced with a situation where they receive
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89 a frame gap (no data at all), and they need to run the EFR decoder with BFI=1 -
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90 but the application doesn't have any frame-bits input. Yet the canonical EFR
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91 decoder requires *some* erroneous frame bits to be fed to it - so what gives?
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92 Our initial approach was to feed the decoder all zeros in the place of codec
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93 parameters - but further analysis reveals that approach to be bad. (To see for
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94 yourself, study the code in d1035pf.c and think what it will do when the input
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95 is fixed at all zeros.) Our new approach is to generate pseudorandom bits for
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96 these pulse parameters, as detailed below.
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97
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98 If you find yourself in the situation of needing to feed BFI=1 with no frame
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99 data bits to the decoder, call the following function in the place of
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100 EFR_decode_frame():
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101
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102 extern void EFR_decode_bfi_nodata(struct EFR_decoder_state *st, int taf,
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103 int16_t *pcm);
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104
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105 This function begins by checking the internal state flag RX_SP_FLAG, indicating
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106 whether the decoder is in speech or comfort noise generation mode. If
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107 RX_SP_FLAG is set, indicating speech state, then the main body of the decoder
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108 will be making use of fixed codebook pulse parameters even for erroneous frames,
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109 and EFR_decode_bfi_nodata() will invoke a PRNG to fill in pseudorandom bits.
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110 If RX_SP_FLAG is clear, then the decoder is generating comfort noise following
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111 reception of a SID, and BFI conditions are fully expected because the
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112 transmitter is expected to be off. In this case EFR_decode_bfi_nodata() feeds
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113 all-zeros parameters to the main body of the decoder, as none of them will be
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114 used.
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115
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116 Stateless utility functions
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117 ===========================
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118
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119 All functions in this section are stateless (no encoder state or decoder state
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120 structure is needed); they merely manipulate bit fields.
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121
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122 extern void EFR_frame2params(const uint8_t *frame, int16_t *params);
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123
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124 This function unpacks an EFR codec frame in ETSI TS 101 318 RTP encoding (the
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125 upper nibble of the first byte is NOT checked, i.e., there is NO enforcement of
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126 0xC signature) into an array of 57 (EFR_NUM_PARAMS) parameter words for the
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127 codec. int16_t signed type is used for the params array (even though all
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128 parameters are actually unsigned) in order to match the guts of ETSI-based EFR
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129 codec, and EFR_frame2params() is called internally by EFR_decode_frame().
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130
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131 extern void EFR_params2frame(const int16_t *params, uint8_t *frame);
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132
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133 This function takes an array of 57 (EFR_NUM_PARAMS) EFR codec parameter words
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134 and packs them into a 31-byte (EFR_RTP_FRAME_LEN) frame in ETSI TS 101 318
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135 format. The 0xC signature is generated by this function, and every byte of the
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136 output buffer is fully written without regard to any previous content. This
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137 function is called internally by EFR_encode_frame().
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138
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139 extern int EFR_sid_classify(const uint8_t *frame);
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140
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141 This function analyzes an RTP-encoded EFR frame (the upper nibble of the first
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142 byte is NOT checked for 0xC signature) for the SID codeword of GSM 06.62 and
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143 classifies the frame as SID=0, SID=1 or SID=2 per the rules of GSM 06.81
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144 section 6.1.1.
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145
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146 extern void EFR_insert_sid_codeword(uint8_t *frame);
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147
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148 This function inserts the SID codeword of GSM 06.62 into the frame in the
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149 pointed-to buffer; specifically, the 95 bits that make up the SID field are all
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150 set to 1s, but all other bits remain unchanged. This function is arguably least
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151 useful to external users of libgsmefr, but it exists because of how the original
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152 code from ETSI generates SID frames produced by the encoder in DTX mode.
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153
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154 Parameter-based encoder and decoder functions
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155 =============================================
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156
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157 The EFR_encode_frame() and EFR_decode_frame() functions described earlier in
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158 this document constitute the most practically useful (intended for actual use)
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159 interfaces to our EFR encoder and decoder, but they are actually wrappers around
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160 these parameter-based functions:
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161
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162 extern void EFR_encode_params(struct EFR_encoder_state *st, const int16_t *pcm,
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163 int16_t *params, int *sp, int *vad);
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164
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165 This function is similar to EFR_encode_frame(), but the output is an array of
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166 57 (EFR_NUM_PARAMS) codec parameter words rather than a finished frame. The two
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167 extra output flags are optional (pointers may be NULL) just like with
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168 EFR_encode_frame(), but there is a catch: if the output frame is a SID (which
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169 can only happen if DTX is enabled), the bits inside parameter words that would
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170 correspond to SID codeword bits are NOT set, instead one MUST call
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171 EFR_insert_sid_codeword() after packing the frame with EFR_params2frame(). The
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172 wrapper in EFR_encode_frame() does exactly as described, and the overall logic
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173 follows the original code structure from ETSI.
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174
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175 extern void EFR_decode_params(struct EFR_decoder_state *st,
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176 const int16_t *params, int bfi, int sid, int taf,
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177 int16_t *pcm);
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178
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179 This function is similar to EFR_decode_frame() with the frame input replaced
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180 with params array input, but the SID classification per the rules of GSM 06.81
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181 section 6.1.1 needs to be provided by the caller. The wrapper in
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182 EFR_decode_frame() calls both EFR_frame2params() and EFR_sid_classify() before
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183 passing the work to EFR_decode_params().
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184
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185 State reset functions
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186 =====================
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187
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188 extern void EFR_encoder_reset(struct EFR_encoder_state *st, int dtx);
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189 extern void EFR_decoder_reset(struct EFR_decoder_state *st);
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190
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191 These functions reset the state of the encoder or the decoder, respectively;
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192 the entire state structure is fully initialized to the respective home state
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193 defined in GSM 06.60 section 8.5 for the encoder or section 8.6 for the decoder.
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194
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195 EFR_encoder_reset() is called internally by EFR_encoder_create() and by the
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196 encoder itself when it encounters the ETSI-prescribed encoder homing frame;
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197 EFR_decoder_reset() is called internally by EFR_decoder_create() and by the
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198 decoder itself when it encounters the ETSI-prescribed decoder homing frame.
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199 Therefore, there is generally no need for libgsmefr users to call these
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200 functions directly - but they are made public for the sake of completeness.
130
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201
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202 If you call EFR_encoder_reset() manually, you can change the DTX enable/disable
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203 flag from its initial value given to EFR_encoder_create() - the new value of
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204 this flag passed to EFR_encoder_reset() always takes effect. There is no
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205 provision for changing this mode within an encoder session without a full reset.